Intensity modulated radiation therapy (“IMRT”) involves changing the size, shape, and intensity of a radiation beam to conform to the size, shape, and location of a patient's tumor. IMRT precisely targets the tumor while sparing surrounding normal tissue. Generally, IMRT is delivered using a radiation therapy system equipped with a radiation source and a linear accelerator (“linac”). The radiation beam exiting the linac travels through a multi-leaf collimator (“MLC”) to change the size, shape, and intensity of the beam.
Prior to the development of the linac, radiation therapy machines used Cobalt-60 (“Co-60”) as the radiation source. Co-60 radiation therapy machines use simple reliable technology that has been available since the 1950s. The Co-60 radioactive source is mounted in the radiation therapy machine. To deliver a dose to the patient, the radiation beam from the Co-60 source is collimated by jaws in a manner similar to that of the linac. Field shaping blocks may also be used to further shape the beam. Co-60 characteristics are stable and predictable, but generally require longer treatment times than the linac. Because the energy of Co-60 radiation is generally lower than that of a linac, Co-60 radiation therapy machines are normally only used to treat relatively shallow diseases such as those of the head and neck. Co-60 radiation therapy machines have not been retrofitted with a MLC, and MLC-based IMRT is not currently available with Co-60 radiation therapy machines.